Chaos was an artificial life-form created by Thanos and an enemy of Captain Mar-Vell.
Thanos initiated his "legacy" following his death (Marvel Two-In-One Annual #2), activating a preprogrammed system within ISAAC, the main computer of his homeworld, one of Saturn’s moons, Titan; this included dominating the Titan-spawned Elysius, imprisoning the native Eternals (at least the threatening members) behind a forcefield, and creating several beings in Life-Baths.
Drax the Destroyer sensed Thanos’ participation in this and attracted Captain Mar-Vell in the process.
One of the beings within the Life-Baths, Chaos, was released by ISAAC following the rebellion of Elysius and the freeing of the captured Titanian Eternals. While the liberated Eternals dealt with ISAAC’s robotic drones, Captain Mar-Vell, Drax, and Elysius faced Chaos. Chaos released its essence of reality around the renegades, and pulled Captain Mar-Vell into the dimension of Tartarus. Using the same life-giving solar energy that reinvigorated the newly freed Eternals against Tartarus, Captain Mar-Vell defeated the God of the Abyss. Shortly after, Elysius and Drax overcame Chaos by entering its destructive heart. Once Chaos was crushed, Mar-Vell returned to reality, in the bowels of Titan’s computer, ISAAC, damaging several image-related relays that gave the computer visual substance in the process.
Alongside Stellarax, Lord Gaea, and ISAAC-Prime (the robotic form of Titan’s computer still under Thanos’ influence), Chaos was resurrected with MORE power than before. They faced off against Captain Mar-Vell and Drax,. After a brief scuffle, ISSAC-Prime transported everybody into his control room which contained the Eternals, incapacitated from oxygen depravation. Chaos blasted Drax, stunning him. Mar-Vell responded by blasting Chaos with his photonic energy, making the construct inert.
Powers / Abilities
Chaos had the ability "to swallow the stuff of Reality," of which it was composed. By expelling this star-stuff outward, Chaos could cover a given area that would slowly increase in size. Within this area, Chaos could form a vortex (probably a variation of a black hole) that could engulf a single, human-sized target. This vortex presumably led to an alternate dimension, more than likely chaotic in nature. It was unknown if various dimensions could be accessed or only the one that was depicted. The "dimension" Chaos accessed was apparently the realm where Tartarus, Titanian God of the Abyss, dwelled, and it contained a gravity well that prevented those drawn into it from escaping.
The longer Chaos expelled its star-stuff, expanding into the universe, the more powerful and larger it grew. As this occurred, Chaos went from stellar-looking to white in color. As Chaos increased in power, it may have been able to engulf even larger targets within its vortex. When resurrected, Chaos’ power had been increased tenfold; it appeared larger in size and could fire blasts of energy. It was unrevealed if the energy blasts were a product of the power increase or not.